1. Field of the Invention
The present invention relates to methods and apparatus for providing a protective lining and impacting surface for equipment used in ore and rock comminution. More particularly, the present invention relates to a new and improved liner assembly and mounting apparatus for providing the shell of an ore grinding mill with a liner having desired metallurgical properties.
2. The Background of the Invention
In commercial mining operations, large autogenous and semi-autogenous mills are often employed to comminute ore removed from the mine. Such mills include a large drum, having a typical diameter of 28 feet and a length of 12 feet. In operation, ore is fed through a trunnion into the feed end of the drum while the drum is being rotated about a central axis. As the drum rotates, the ore is comminuted by being subjected to both continuous-pressure and impact mechanisms. The ore is then removed from the opposite, or discharge end of the mill.
These autogenous and semi-autogenous mills are typically intended for continuous operation. However, because ores being comminuted in the mill may be hard and highly abrasive, the drum will quickly wear out unless some provision is made to protect the drum from wear while the mill is in operation. Replacing the drum not only would cause a serious disruption in the operation of the mill, but would result in such a significant expense that the use of such a mill would be impractical.
The universally accepted solution for protecting the drum from wear is to employ a liner which may be mounted onto the cylindrical sections of the drum, or the "shell" of the mill. In recognition of this necessity to include a liner, when the drums are manufactured, a series of rows of mounting holes are drilled into the shell of the mill. A series of liner segments may then be mounted onto the shell of the mill utilizing these mounting holes, thereby virtually completely covering the shell of the mill. These mounting holes are typically spaced in the axial direction (i.e., along the axis of rotation of the mill) approximately 12 to 24 inches apart.
After a period of use, the liner segments are worn to the point that they must be replaced. In order to reduce to a minimum the amount of down time of the mill associated with the replacement of liner, liner design has been directed towards facilitating rapid replacement of the liner.
It takes virtually the same amount of time to replace a large liner segment as it takes to replace a small liner segment. Thus, the trend in liner design has been to make liner segments as large as possible, resulting in fewer liner segments to replace. For example, by doubling the size of the liner segments, the number of liner segments which must be replaced is reduced by half. This results in a corresponding reduction in time required to replace the liner.
Because of the weight of the liner segments, special equipment is employed to lift the segments and place them in position for mounting to the shell of the mill. This "liner handler" is always used to support the liner segments during mounting. Thus, the increased weight associated with employing larger liner segments results in a negligible increase in the difficulty of replacing the liner.
In addition to being advantageous to mill operators in reducing the amount of down time of the mill during replacement, large liners also represent a significant economic advantage over smaller liners to liner manufacturers. A significant factor in determining the price which is charged for such liners is their weight. Liners are usually priced by charging a predetermined amount per pound of material.
Because such liners are made by casting, a liner manufacturer may double the poundage of sellable material produced in one mold simply by doubling the size of the liner. It is not uncommon to produce a liner with one casting which results in several thousand pounds of material which is ready to sell. As is the case when installing the liner, casting amount of work involved. Thus, when producing liners having half the size, twice as much work is involved by the manufacturer to produce the same dollar volume of product.
Because of the enormous size and weight of most ore grinding mills, the size limit of steel plate which is available, the capacity of metal forming machines, and the transportation limitations which arise when dealing with such machinery, it is necessary to manufacture the mills in several sections which may be assembled at the mill site. The mills are typically made of cylindrical quadrants having flanges extending from their perimeter for mounting to one another. By representative example, when constructing the mill, the cylindrical quadrants are mounted lengthwise to each other to form a cylinder. Several cylinders may be mounted to each other to achieve the desired length of mill. End pieces may then be mounted to the ends of the cylinder to enclose the mill
The joints along the circumference of the drum represent the weakest structural points in the drum. To compensate for this weakness, liners mounted inside the drum may be mounted such that they span these joints and are secured to the drum on both sides of the joints. Such a liner, therefore, serves a dual purpose; it provides a hard material used in comminuting the ore and it reinforces the structure of the drum, thereby lending stability to the mill.
From the foregoing, it can be seen that significant economic forces have dictated that the size of liners employed to protect the shell of the mill be as large as possible. Additionally, the use of large liners has been preferred because their size enables the liners to be used to reinforce the joints of the cylindrical quadrants which are mounted together to form the mill.
Replaceable impact surfaces found in other comminution equipment also tend to be large for many of the same reasons as described above. For example, the blow bar used in a rock impact crusher is preferably made of one piece, thereby keeping to a minimum the time involved to replace the blow bar. Additionally, manufacturing of the blow bar is facilitated if only one casting must be performed to produce the blow bar.
The use of large impact surfaces, however, does present various difficulties. For example, mill shell liners are preferably made of a material which is highly abrasion resistant in order to withstand virtually continuous contact with hard and highly abrasive ores. Additionally, the liner must be impact resistant so that it does not rapidly disintegrate due to brittle failure during operation of the mill.
Because the liner must have a high hardness, it is not feasible to machine the liner segments. Use of a material which would be machinable with conventional equipment would necessarily require use of a material which would not have sufficient hardness for use as a liner. Thus, manufacturing liner segments of a castable material is the only economically viable method of manufacture.
Although the properties of hardness and toughness are, to a large extent, exclusive of each other, a suitable combination of hardness and toughness may be obtained by heat treating the liner. An example of a material ideally suited for this application would be martensitic white iron or martensitic steel.
High hardness is obtained in the liner segment through heat treatment. After the liner segment has been cast, it is heated and allowed to "soak" at a given temperature for a period of time, thereby forming austenite. Following the austenite formation, the segment is rapidly cooled, or "quenched," to form martensite. The quenching must occur fast enough to avoid transformation to pearlite or bainite.
The primary difficulty which arises when attempting to quench a large casting to form a martensitic microstructure throughout the liner segment is that because of the thickness of the liner the rate of heat loss may not be sufficient to avoid transformation to another microstructure. This frequently results in the formation of a martensitic microstructure at the surface of the casting with other, softer microstructures being formed at the core. Additionally, the slower rate of solidification associated with the larger casting will produce a product having a larger grain size than a smaller casting, thereby adversely affecting the hardness of the final product.
One of the hazards of rapid quenching is the possibility of distorting and cracking the liner segment. As the surface portions of the liner segment pass through the martensite transformation, they will initially expand as the temperature in that portion of the liner drops and martensite is formed. The remainder of the liner is still austenite, soft and hot, and follows the expansion. Then, as the rest of the liner passes through the martensite transformation and the associated expansion, the surface portions of the liner, which are hard, brittle martensite, will frequently crack.
The manufacturing process must, therefore, be carefully monitored to ensure that the temperature gradient within the liner segments stays within acceptable limits, thereby avoiding cracking of the liner segments during quenching. Even though the liner segments may not crack, uneven quenching may set up residual stresses within the liner segments which will decrease liner life. These difficulties associated with the production of liner segments having a martensitic microstructure obviously increase the cost of manufacture of such liner segments.
Thus, one of the primary disadvantages associated with the production of martensitic liner segments is that it is difficult to obtain the same degree of hardness in the core of the liner segment as at the surface. In operation, once the hard surface of the liner becomes worn, the remainder of the liner, which does not enjoy the same degree of hardness as the surface, will quickly wear. This obviously decreases the operational time of the mill between replacement of liners.
Another feature which adds to the difficulty of casting the liner segments is that bolt holes must be provided in the segments through which a bolt may be inserted to mount the liner segments to the mill shell. When preparing a mold which will cast a liner having a hole in it, an insert must be provided in the mold to form the hole. As the liquid metal is poured into the mold, it forms swirls and curls around the insert which results in a weak zone at that location. When the part fails, it usually fails at the hole.
In recognition of the weak zone which exists at the hole in the liner, most liners are designed such that the hole is not in the primary wear section of the liner. The liner is provided with a recessed area which includes the hole. The disadvantage with this configuration is that the wear section, or portion of the liner exposed to the ore stream, must necessarily be smaller to provide for the recessed portion containing the hole.
Attempts made to attach the mounting bolt to the liner thereby eliminating the through hole have failed because of problems associated with removing the liners. A typical liner segment may be mounted to the shell of the mill with several bolts. If an attempt is made to remove the liner without first removing the bolts, the liner will bind because of the difficulty of evenly pulling the liner bolts out of the holes in the shell of the mill.
Thus, the usual practice for removing a liner segment is to first remove at least all but one of the bolts by removing the nut on the outside of the shell and pushing the bolt through the hole in the liner. The liner segment may then be broken loose and removed without any binding.
One proposed solution to the manufacturing problems encountered when attempting to produce a large liner segment having a hardness sufficient for use in a mill is to include one or more alloys in the casting. It is generally recognized that alloys may be used to produce a material having desired mechanical properties when the physical parameters of the casting prevent the material from being heat treated to attain those properties. Increasing the amount of alloys in the casting enables a liner having a coarser grain size to be produced with the same hardness as a non-alloyed material having a finer grain. Thus, alloys permit the successful hardening of many complex designs that could not otherwise be produced.
However, a serious disadvantage to the use of a substantial amount of alloys is their high cost. Although alloys may enable a desired hardness to be achieved in a complex design, the increased cost associated with the use of alloys may render the use of such alloyed liners impractical for many applications.
Another means employed by the prior art to achieve a liner assembly having a hard surface is to use a composite liner assembly. A composite liner is a liner assembly which employs a tough material for the primary structure of the liner coupled with one or more inserts or segments formed from a highly abrasion-resistant material which comprises a secondary structure. The tough primary structure is attached to the hard secondary structure in such a manner that the hard inserts or segments are exposed directly to the ore fragments.
Composite liner assemblies are designed primarily for use in rod mills where there is no point contact. In ball mills and autogenous mills where there is a substantial amount of point contact with the liners, composite liners are not effective because the hard inserts only cover approximately 30 percent of the surface area of the shell of the mill.
Another disadvantage to such composite liner assemblies is that they are geometrically complex and utilize complicated mounting mechanisms. Thus, composite liner assemblies are frequently expensive to manufacture and, because of their many parts, are difficult to install. Additionally, when the hard secondary material eventually breaks away due to its brittleness, the hard inserts or segments must immediately be replaced before the primary structure is irreparably damaged by the abrasive action of the ore.
Because the primary structure serves no purpose other than as a mounting mechanism for the hard secondary structure, it adds weight to the already heavy mill without providing a corresponding increase in crushing efficiency.
It will be appreciated, therefore, that what is needed in the art are methods and apparatus for covering the shell of an ore grinding mill with a liner which may be easily and inexpensively installed and replaced.
It would be a further enhancement in the art if such liners could be manufactured such that the microstructure of the liner could be controlled during heat treatment, thereby producing a liner having the same microstructure throughout (such as a martensitic microstructure) and substantially the same grain size throughout.
Indeed, it would be yet a further advancement in the art if such a liner could be heat treated during the manufacturing process such that the risks of breaking the liner and establishing significant residual stresses within the liner are substantially eliminated.
It would be an additional enhancement in the art if such liners could be provided with a mounting mechanism which eliminate the necessity for through holes in the liner, thereby avoiding weak zones in the wear section of the liner.
It would also be an advancement in the art if such a liner could be manufactured without employing significant amounts of expensive alloys.
It would be an additional advancement in the art if such liners could be manufactured without employing a composite liner assembly having a tough material as a primary structure and hard material for a secondary structure, thereby eliminating the complex, intricate configurations associated with such liner assemblies and providing a liner assembly having a lower weight than such composite liner assemblies.
Such methods and apparatus are disclosed and claimed herein.